A question to ask is the medium being utilized for your ISP. Do you have coax, copper pair (DSL), WISP, Satellite, etc.
Knowing what the medium is and or the company providing your access is helpful as well. Easier to troubleshoot/narrow down.
Remember, 2 rules for troubleshooting:
1.) Always blame the distant end. It's never your equipment's fault.
2.) If rule 1 didn't work then start from layer 1 of OSI and work your way up to 7.